SOIL PREPARATION

Soil aeration (ratio of oxygen to carbon dioxide) inside ridges also improved as a result of better internal drainage compared to the flat un-ridged control (Myburgh & Moolman, 1991b). These authors also reported a decrease in leaf water potential and an increase in stomatal resistance on the control plots at oxygen levels below 16 %. Such symptoms are typical responses to inadequate oxygen supply (Glinski & Stepniewski, 1985; Nicolás et al ., 2005). The indications of physiological stress remained until the water table dropped to below 60cm later in the season. Soil temperature in the upper part of ridges, i.e. above 30 cm, was found to be significantly higher than in un-ridged soil during the early part of the season. This trend continued throughout the season (Myburgh & Moolman, 1993). Albeit to a limited extent, soil temperatures in ridges even exceeded 30ºC during the warm later part of the season. Improved soil properties such as increased soil volume having lower bulk density, as well as better drainage and aeration with ridging, culminated in more rapid root growth and higher root lengths of one-year-old Chardonnay/ Teleki grapevines (Eastham et al ., 1996). They associated the improved root development on raised beds with greater water extraction and possibly also nutrient uptake. In South Africa, root studies on Chenin blanc/99R showed a less clear benefit of ridging since the number of roots in the topsoil was actually lower than in flat un-ridged vineyards (Myburgh, 1994). The majority of deeper roots on the un-ridged treatment plots, however, died back during the annual cycle of waterlogging while the roots on the ridged soil remained active as a result of better internal drainage and aeration. Under dryland conditions, ridging tended to increase grapevine yield and shoot growth compared to un-ridged soil (Myburgh, 1994). Pruning mass was not affected by either ridge height (400 mm and 600 mm) or by ridge width. Myburgh (1994) did, however, establish a relationship between grapevine yield and the surface/volume ratio of ridges. Based on these relationships, he found yield losses when the soil surface to soil volume ratio was less than 0.6, and consequently he recommended double-row ridges of less than 400 mm high and 1.5 m wide at the crest. Such dimensions can be readily obtained if the centrelines of ridges are 4.5 m apart and the topsoil is deeper than 300 mm. Single-row ridges caused higher soil temperatures and more intense drying of the soil, and as a result the soil surface to volume ratios below 1.0 already reduced grape yield. In practice this would mean that single-row ridges should have approximately 500 mm wide flat crests, be lower than 400 mm, and be at least 1.0 m wide at the base. These recommendations regarding ridge dimensions are based on one experiment and are probably not universally valid, but it gives a clear starting point for growers and researchers who wish to pursue this vineyard practice. Furthermore, the above-mentioned recommendations do not apply to irrigated vineyards.

64 | SPECIAL SOIL PREPARATION STRUCTURES